1. Technical Field
This invention relates generally to a semiconductor laser drive circuit, more particularly, to a semiconductor laser drive of use in an optical transmitter, having immediate applications in commercial CATV systems.
2. Related Art
Commercial terrestrial Cable Television (CATV) systems typically utilize optical links (e.g., fiber-optic cable, with RF transmitter-receiver pairs) to carry radio frequency (RF) digital signals over long distances. The light-producing component is typically a semiconductor laser diode operating in forward bias mode.
The forward bias of the semiconductor laser diode is maintained by a DC current generally referred to as the bias current (IB). A semiconductor laser emits light when a diode current (ID) flowing through it exceeds a threshold value (ITH), the value of which may vary from diode to diode, and may vary in a particular diode during operation due to operational conditions such as temperature. Generally, the bias current IB is controlled so as to establish and maintain the relation IB≈ITH. Radio frequency (RF) signals expressed as a RF modulated electrical current (IP) are superimposed upon the DC bias current (IB,) through the laser diode to obtain modulated light signals that will carry the RF information long distances (e.g., through a fiber optic cable physically and optically coupled to the laser diode). The current flowing through the laser diode (ID) during operation is therefore equal to the sum of the currents IP plus IB. The amperage of bias current IB can be larger or smaller than the maximum amplitude of RF modulated current IP.
FIG. 1 is derived from FIG. 1 of U.S. Pat. No. 5,563,898, issued to Ikeuchi, (incorporated herein by reference except for the parenthetical references to xe2x80x9cgroundxe2x80x9d in FIGS. 11 and 13 thereof) and is representative of the related art""s approach to supplying bias current (IB) and RF modulated electrical current (IP) to the laser diode 11. The related art generally teaches that a distinct drive-current (IP) supply unit 12 (powered by the full supply voltage VDD) plus a distinct bias-current (IB) supply unit 13 (connected between GND and the laser diode 11), are both necessary to provide and regulate the current ID (e.g., ID=IP+IB) through the laser diode 11. The related art generally teaches that the such a distinct bias-current (IB) supply unit 13 should be connected in parallel with the drive-current (IP) supply unit 12 between a terminal (Node2) of the laser diode 11 and ground. Thus, in the related art, the electrical power that is consumed in the drive-current (IP) supply unit 12 to provide bias current (IB) to the laser diode 11 is essentially dissipated as waste heat, and does not power the operation of the drive-current (IP) supply unit 12. Therefore, the semiconductor laser drive circuits of the related art are not as power-efficient as the present invention discloses that a semiconductor laser drive circuit can be. There is a need for a power efficient method and apparatus for transmitting RF signals through an optical medium.
The present invention overcomes the disadvantages of the semiconductor laser drive circuits of the related art. The disclosed invention has applications in cable television (CATV) distribution networks as well as in many other applications involving fiber-optic transmission of RF signals. RF digital signals that may be transmitted using the inventive semiconductor laser drive circuits and methods may include QAM (e.g., M-ary Quadrature Amplitude Modulated), Baseband, QPSK and other types.
The present invention exploits the observation that a typical semiconductor laser diode, such as laser diode 11 shown in FIG. 1, typically operates with a significant bias current (IB) but with a forward bias voltage drop (VFB) across its terminals (i.e., measured between Node 1 and Node 2) that is very small compared to supply voltage VDD needed to operate RF components such as a drive-current supply unit (e.g., an RF Amplifier). The present invention further exploits a discovery that such RF components generally can operate at a reduced supply voltage VD that is less than the typical full supply voltage VDD (e.g., Vdd=24 volts), and that VD may be equal to VDD minus VFB. Accordingly, the invention provides semiconductor laser drive circuits wherein the DC bias current (IB,) through the laser diode 11 is used to power the RF components of the laser drive circuits (e.g., in 12) rather than being dissipated as heat (e.g., in 13), and thus eliminating the need for a distinct bias-current (IB) supply unit 13 as was required in the related art circuit shown in FIG. 1.
In a first general aspect, the present invention provides an apparatus comprising: a laser diode having a DC bias-current (IB) and a radio frequency (RF) modulated RF drive-current (IP); and a drive-current supply unit adapted to pass the DC bias-current (IB) and the RF drive-current (IP) through said laser diode, wherein said drive-current supply unit includes at least an output stage of a radio frequency (RF) amplifier (e.g., a Hybrid) that passes the RF drive-current (IP) and at least a portion of the DC bias-current (IB).
In a second, more particular, aspect the present invention provides an apparatus comprising: a laser diode having a DC bias-current (IB) and an RF drive-current (IP); and a push-pull (Hybrid) RF amplifier connected in series with the laser diode between the supply voltage (VDD) and ground, wherein the push-pull (Hybrid) RF amplifier passes the RF drive-current (IP) and at least a portion of the DC bias-current (IB).
In a third general aspect, the present invention provides an apparatus comprising: a laser diode having a DC bias-current (IB) and an RF drive-current (IP); a drive-current supply unit for passing the DC bias-current (IB) and the RF drive-current (IP) through said laser diode, wherein said drive-current supply unit includes at least an output stage of a radio frequency (RF) amplifier that passes the RF drive-current (IP) and at least a portion of the DC bias-current (IB); and a diode-bypass current (IBP) path, wherein the diode-bypass current (IBP) path is connected in parallel with the laser diode, and the diode-bypass current (IBP) path is connected in series with the drive-current supply unit.
In a fourth general aspect, the present invention provides a semiconductor laser drive circuit for driving a laser diode having a DC bias-current (IB) and an RF drive-current (IP), comprising: circuitry for passing the DC bias-current (IB) through said laser diode, wherein said circuitry includes at least an output stage of a radio frequency (RF) amplifier that passes at least a portion of the DC bias-current (IB),
In a fifth general aspect, the present invention provides a semiconductor laser drive circuit for passing a DC bias-current (IB) and an RF drive-current (IP) through a laser diode, the circuit comprising radio frequency (RF) circuitry adapted to RF modulate the RF drive-current (IP) and further adapted to pass the DC bias-current (IB).
In a sixth general aspect, the present invention provides an apparatus comprising a laser diode having a DC bias-current (IB) and an RF drive-current (IP); and radio frequency (RF) circuitry that passes at least a portion of the DC bias-current (IB) and the RF drive-current (IP).
In a seventh general aspect, the present invention provides an optical transmission system comprising: an optical signal transmitter for transmitting RF signals, the transmitter including a laser transmitter bias circuit; an optical signal receiver; an optical link medium being operatively connected between the optical signal transmitter and the optical signal receiver; wherein the laser transmitter bias circuit includes: a laser diode having a DC bias-current (IB) and an RF drive-current (IP); a drive-current supply unit adapted to pass the DC bias-current (IB) and the RF drive-current (IP) through said laser diode, wherein said drive-current supply unit includes at least an output stage of a radio frequency (RF) amplifier that passes the RF drive-current (IP) and at least a portion of the DC bias-current (IB).
In a eighth general aspect, the present invention provides a method for communicating radio frequency (RF) informational signals through an optical link medium, said method comprising: providing an optical signal transmitter for transmitting RF signals, the transmitter including a laser transmitter bias circuit, wherein the laser transmitter bias circuit includes: a laser diode having a DC bias-current (IB) and a radio frequency (RF) modulated RF drive-current (IP); and a drive-current supply unit adapted to pass the DC bias-current (IB) and the RF drive-current (IP) through said laser diode, wherein said drive-current supply unit includes at least an output stage of a radio frequency (RF) amplifier that passes the RF drive-current (IP) and at least a portion of the DC bias-current (IB).
The foregoing and other features and advantages of the invention will be apparent from the following more particular description of embodiments of the invention.